DD4 has a few sets. At first she builds them to spec and plays with them. She likes to break them apart and rebuild a few times. After she's done that a couple of times, she'll use the pieces to make her own creations.

She also has a gigantic bag of loose lego that DH saved from his childhood which she uses to free play with.

She's obsessed with lego in general and usually plays with them for a couple of hours everyday.